A thin film transistor active matrix liquid crystal display (TFT-LCD) is a advanced product in the current liquid crystal display (LCD) market. With development of thin film transistor process, the TFT-LCD has become a popular product in the current LCD market. FIG. 1 is an equivalent circuit diagram of sub pixel of an existing panel, which comprises gate line Gn, data line D, TFT, parasitic capacitor Cgd between gate and drain of TFT, parasitic capacitor Cgs between gate and source, parasitic capacitor Cds between drain and source, two terminals of the liquid crystal capacitor C1c are respectively connected to a common electrode C and a pixel electrode P, and one terminal of the storage capacitor Cs is connected to the pixel electrode P and the other terminal is connected to the next gate line Gn+1.
With a broadly adopted architecture at present where a common electrode voltage VCOM is fixed, when a voltage on gate line varies, correctness of voltage on pixel electrode is affected by a parasitic capacity Cgd between the gate and the drain such that a DC component-coupling voltage is applied on the pixel electrode, thus because of characteristics of liquid crystal molecules, after a TFT-LCD drives specific still image for a long while, the pixel electrode is applied with a DC component for a long while, then graph of the previous image will not leave when transforming to another images, which makes an image sticking. The reason for the image sticking generation is presence of a coupling voltage, which cause non-symmetry for positive/negative polarity of a pixel electrode voltage.
FIG. 2 is a waveform diagram illustrating the change of real pixel electrode voltage, which reflects pixel electrode voltage change due to effect of a coupling voltage, wherein Vg is a gate voltage, Vp is a pixel electrode voltage, VCOM denoted by solid line is a real VCOM value, dashed line is an ideal pixel electrode voltage without coupling voltage, solid line is a real pixel electrode voltage due to influence of a coupling voltage, and VCOM denoted by solid line is a real common electrode voltage applied upon the common electrode. As illustrated by FIG. 2, positive/negative polarity of the real pixel electrode voltage is not symmetrical with respect to the real common electrode voltage due to presence of the coupling voltage, and VCOM denoted by the dashed line is an ideal common electrode voltage which can make the positive/negative polarity of the real pixel electrode voltage symmetric.
When a gate of a TFT on the panel is turned on, a coupling voltage could be generated on a pixel electrode. Since a source and a drain of the TFT are turned on, a source driver would begin to charge the pixel electrode, then charges on the parasitic capacity Cgd, the storage capacitor Cs and the liquid crystal capacitor C1c can be maintained by applying with a voltage on the source. Therefore, even if the pixel electrode voltage is not correct at the beginning (due to effect of the coupling voltage), the source driver charges the pixel electrode voltage to a correct voltage, such that no substantial impact is generated. When the gate of the TFT is turned off, however, no current source provides charges for the parasitic capacity Cgd, the storage capacitor Cs and the liquid crystal capacitor C1c, and the source driver has stopped charging the pixel electrode, such that the charges on those three capacitors are re-distributed (capacitors Cgs and Cds are not involved in the above charge re-distribution since one terminal of them is connected with the source of the TFT). A voltage drop (30-40 Volts) generated when a gate driver is turned off is fed back onto the pixel electrode via the parasitic capacity Cgd, such that a voltage drop as to a coupling voltage is generated with respect to the pixel electrode voltage, and thus correctness of gray scale displaying could be influenced. And such a coupling voltage behaves in a way not same with that of the coupling voltage generated when a gate line is turned on which only makes an impact once, and the voltage drop of such a coupling voltage would continue to influence the pixel electrode voltage till the gate driver is turned on again for the next time since the source driver has stopped charging/discharging the pixel electrode. Therefore, human's eye can easily sense the influence made by the coupling voltage onto gray scale of a displaying image.
With respect to a current design employing a fixed common electrode voltage, the coupling voltage would introduce non-symmetry as to positive/negative areas of the pixel electrode voltage (it is a positive polarity if Vp>VCOM, a negative polarity if Vp<VCOM), so an image sticking is generated. Even if the real common electrode voltage is adjusted according to a specific coupling voltage generated at a time so as to make it consistent with the ideal value (referring to FIG. 2, the common electrode voltage before adjusting is denoted by the solid line, and the one after adjusting is denoted by the dashed line), there could be a difference between the real common electrode voltage and the ideal value for the next time since the coupling voltage on the panel may vary when a fixed image is displayed on the liquid crystal panel for a long while or the panel stays in an environment with high humidity and high temperature, and the image sticking may be generated as well. Therefore, if only one fixed common electrode voltage is inputted or the real common electrode voltage is adjusted according to a specific coupling voltage generated at a time, there is a difference between the real common electrode voltage and the ideal common electrode voltage and the influence of the coupling voltage can not be removed, such that the image sticking is generated.